This invention relates to a method and apparatus for comminuting a wide variety of materials to an extremely fine size using a re-entrant circulating stream jet mill and a discrete but interconnected and interdependent jet and rotating anvil-jet impact mill. For the purposes described herein, extremely fine size means particles averaging below 20 microns and, more specifically, below 10 microns including certain specified end products averaging less than 2 microns.
Heretofore, various materials have been comminuted to intermediate average particle size using hammer mills with integral whizzer classifiers, ring roll and bowl mills with integral whizzer classifiers or wet bowl mills. The difficulty with wet bowl mills is that certain material tends to agglomerate when drying and, therefore, requires subsequent breaking up. Regardless, the mill best able to comminute particles to the lowest micron size has been the jet mill of which there have been three basic forms. The first such mill is disclosed in Luckenbach, U.S. Pat. No. 697,505 which uses opposed jet mills. Condensation and classifying problems, however, limited the usefulness of the Luckenbach mill. The next jet mill of any consequence was of the type disclosed in U.S. Pat. No. 1,935,344 and several kindred patents. This type of jet mill is in use today as the so-called "Majac Jet Pulverizer". It is described in its present form in the Fifth Edition of Perry's Chemical Engineering Handbook, Chapter 8, page 44. The Majac mill uses a whizzer type classifier making it superior in operation to the typical jet and anvil mill, such as disclosed in U.S. Pat. No. 2,487,088.
The third and most universally used of all jet type communiting mills for low micron size is the re-entrant circulating stream jet mill disclosed in U.S. Pat. No. 2,032,827 and its prodgeny. This mill is often referred to as a Micronizer.
There have been many attempts to improve upon the Micronizer since it was first introduced, and some of them have been significant improvements. The most successful improvement to the Micronizer for comminuting a wide range of materials is disclosed in U.S. Pat. No. 3,688,991, particularly in columns 7 and 8 and FIGS. 7 and 8. This mill, called the Cyclo-Jet, has been successfully used for the past 10 years for comminuting a wide range of materials with reduced consumption of energy. For example, the mill has been used to comminute talc at an energy cost of approximately 50% of the operating cost of previously known jet type mills.
Not withstanding the success of the Cyclo-Jet, it too has inherent limitation in both its design and operation. To understand these limitations, it is important to note that the Cyclo-Jet is basically a combination of a rotating anvil-jet impact mill and a circulating stream jet mill. Circulating stream jet mills are complex devices despite their apparent simplicity of construction. There are numerous variables which must be balanced or coordinated to achieve the optimum comminution. Some of these variables include the diameter and peripheral height of the grinding and classifying chamber, the number and size of nozzles which in turn depend upon the kind of gas and its pressure, and the angle of the jet stream emitted by the nozzles. Another variable is the shape of the lateral walls which close the grinding and classifying chamber. These can be parallel plates or they can be axially divergent. If the latter, then consideration must be given to the angle of divergence and the radial extent thereof. Other factors include the location and structure of the material feed apparatus as well as the specific gravity and other characteristics of the matrial being fed.
The advantage of the Cyclo-Jet mill disclosed in U.S. Pat. No. 3,688,991 is that the rotating anvil-jet section most efficiently comminutes coarser fractions of material while the circulating stream jet section is best for providing the finest grinding and classification. The problem is that that these two types of mill are not fully compatible when operating in a common grinding and classification chamber. For example, in many instances it is desirable to provide a circulating stream jet mill with diverging walls. This is not possible if the same mill has to include a shaft turning a rotor with anvils at its periphery. Still further, there are problems with supporting the rotor at the distal end of a shaft, as is necessitated by the presence of a centrally positioned outlet in a Micronizer. Experience has shown that distally mounted rotors tend to become unstable, particularly at high rotative velocities. Yet, experience has shown that such velocities are desirable.
As indicated above, it is advantageous in some instances to provide diverging side walls for the grinding and classifying chamber of a circulating stream jet mill. This is not to say that parallel side walls do not have their uses. Parallel walls are preferred for grinding various precipitates and spray dried agglomerates. However, by and large diverging side walls are to be preferred.
It is has been known for some time that the operation of a mill is improved by increasing the quantity of gas swirling within the grinding and classifying chamber. This is accomplished by providing diverging side walls. The amount of divergence is least for high specific gravity materials and greatest for low specific gravity materials. The amount of divergence is greatest at or adjacent the axis of the mill. FIG. 2 of U.S. Pat. No. 3,559,895 is a good illustration of a preferred form of circulating stream jet mill with diverging lateral walls.
A Cyclo-Jet type mill such as illustrated in U.S. Pat. No. 3,688,991 has not yet successfully been provided with diverging walls and rotating disc and anvils. This inventor built a structure similar to the mill illustrated in FIG. 2 of U.S. Pat. No. 3,559,895 but with a 24 inch dish shaped rotor like the lower lateral wall in FIG. 2 of that patent. The rotor proved to be dynamically unstable, probably because of the great peripheral weight of the anvils. The rotor was modified to make it hollow approximately four inches radially outward from the shaft, but the vibration cracked the welds and the experiment was abandoned.
In addition to unstable rotors, the Cyclo-Jet mill does not permit the use of anvils fixed on the peripheral wall of the chamber. This inventor tried welding ribs on the inner periphery of the chamber. This is sound practice in some impact mills, but is was counterproductive in the Cyclo-Jet mill because it interfered with classification of the finally comminuted product. The static anvils slowed the rotational velocity of the material laden gases at the periphery thus nullifying the increased gas velocity generated by the anvils on the rotor. See, for example, column 4, lines 3-15 of U.S. Pat. No. 3,348,799 for a description of how the anvils on the periphery of a rotor assist in increasing the overall efficiency of a mill.
In general, rotating anvil-jet impact mills comminute to finer particle size at higher rotor velocities. Although all materials do not require the same anvil speed for a desired average particle size, velocity tests using a 24 inch rotor with anvils on the periphery are significant. A Cyclo-Jet mill was modified so that the rotor could be turned at velocities of 2,000, 3,600, 4,000 and 4,500 revolutions per minute. The product quality increased with increasing rotor velocity. However, the rotor showed signs of instability at 4,500 r.p.m. Today, approximately 4,000 r.p.m. is about the maximum rotor velocity in commercial use for a 24 inch mill. Of course, higher velocities are possible for mills having smaller rotor diameters. The maximum speed for a six inch mill is 12,000 r.p.m.; for a 12 inch mill it is 8,000 r.p.m.; and a 20 inch mill can operate at 4,500 r.p.m. These rotational velocities convert into peripheral (anvil) velocities of approximately 18,000 feet per minute for the six inch mill, 25,000 feet per minute for a 12 inch mill, 23,000 feet per minute for a 20 inch mill, and 25,000 feet per minute for a 24 inch mill. These velocities cannot be achieved using a dished rotor with anvils, because as indicated above with respect to the modified mill of FIG. 2 of U.S. Pat. No. 3,559,895, the rotor becomes highly unstable at those speeds.
Yet another problem with the Cyclo-Jet is that it cannot take advantage of improvements which have been made in the manner of feeding material into the circulating stream jet part of the mill. For a discussion of the problems encountered in feeding this type mill, see U.S. Pat. No. 4,018,188 and 4,189,102. FIG. 1 of the latter patent illustrates a conical chamber dependent from one of the side walls of the grinding and classifying chamber. This conical chamber creates a vortex of material and injection gas which is merged with the gaseous vortex within the grinding and classifying chamber at approximately the jet tangent circle. In this manner the fluid carrying the feed material assists rather than detracts from the velocity of the main vortex. See also, U.S. Pat. No. 4,428,387 which illustrates modifications of the same concept.
Again, the Cyclo-Jet cannot be adopted to use these improved material feed means because the presence of both the rotor and the feed means would leave no place for a centrally positioned outlet.
The present invention is directed to providing a method and apparatus for communuting materials to extremely fine sizes while overcoming the aforesaid limitations found in the Cyclo-Jet, yet retaining all of its benefits.